SAW filters are widely utilized as IF filters in television receivers and filters in various kinds of communication apparatuses. But they are usually used as a fixed band filter.
FIG. 7 shows a perspective view indicating the general form of the usual SAW filter, in which the reference numeral 1 is a piezoelectric substrate; 2' indicates comb-shaped electrodes constituting an input transducer 2; and 3' indicates comb-shaped electrodes constituting an output transducer 3.
Since the band of such a filter is determined by the form and number of pairs of comb-shaped electrodes, it is a constant proper to the SAW filter and it is not possible to vary it. However, variable band filters are strongly desired for communication apparatuses, in which a frequency band varies with time, and for various sorts of multi-channel communication apparatuses.
FIG. 8(a) is a top view illustrating a representative variable band SAW filter, in which corresponding items are indicated by the same reference numerals as in FIG. 7; 4 represents a switching circuit; 5 represents the input terminal; and 6 represents the output terminal. In the SAW filter indicated in FIG. 8(a) a plurality of SAW filters are mounted on a substrate so that the bands of the filters are adjacent to each other, and one of the bands is selected by switching an external circuit 4. FIG. 8(b) shows five curves representing the relation between the frequency and output, when the contact of the switch in the switching circuit is respectively positioned at A, B, C, D and E in FIG. 8(a).
For such a prior art filter an external circuit 4 for switching is inevitable, what is problematical with respect to the manufacturing cost and space saving. In addition, it has as a drawback that the freedom for the shape of the pass band is small, because it is controlled only by switching to select a single output transducer.
Further, a SAW delay line is used for signal processing in radar devices and as a delay line in a SAW oscillator. In particular, development of a device for which the delay time can be varied is desired, because such a device can be applied to a variable frequency oscillator and to a ghost canceler for a television receiver.
FIGS. 20 and 21 illustrate representative prior art SAW devices used as variable delay lines, in which the reference numeral 1 represents a piezoelectric substrate; 2' indicates comb-shaped electrodes constituting the input transducer 2; 3A, 3B, . . . , are comb-shaped electrodes constituting output transducers 3; 4 is a switching circuit; 5 is the input terminal; and 6 is the output terminal.
FIG. 20 illustrates a so-called delay line with taps, for which the area of the substrate may be small, but which has as a disadvantage that interference between different taps is produced due to reflection of a SAW at each of the taps (output comb-shaped electrodes 3A, 3B, . . . ). On the other hand, for the device illustrated in FIG. 21, although interferences occurring between the output comb-shaped electrodes 3A, 3B, . . . is small, a disadvantage is that a large piezoelectric substrate 1 is necessary. Further, for both the devices an external circuit 4 for switching is necessary and thus they are problematical with respect to manufacturing cost and space saving.